Proppants are used to keep open fractures created by hydraulic fracturing of a subterranean formation, e.g., in an oil or gas bearing strata. Typically, the fracturing is performed in a subterranean formation to increase oil or gas production. Fracturing is caused by injecting a viscous fracturing fluid at a high pressure into the well. As fractures are formed, a particulate material, referred to as a “propping agent” or “proppant” is placed in the formation to maintain the fracture in a propped open condition when the injection pressure is released. As a fracture forms, the proppants are carried into the well by suspending them in a fluid filling the fracture with a slurry of proppant. Upon release of the pressure, the proppants lodge in the fractures so that the fractures do not close once fracturing pressure is reduced. Using proppants increases production of oil and/or gas from a subterranean formation by providing highly conductive channels through the formation.
The maintenance of these channels provides increased flow of various fluids, e.g., hydrocarbons such as natural gas and oil.
Proppant materials that have been widely used include: (1) particulate sintered ceramics, typically aluminum oxide, silica, or bauxite, often with clay-like binders or other additives to increase the particulate's compressive strength, especially sintered bauxite; (2) natural, relatively coarse, sand, the particles of which are roughly spherical, generally called “frac sand” and (3) resin-coated particulates of (1) and (2), i.e., resin-coated proppant.
Unfortunately, each of these materials (as well as others) has a relatively high density (high specific gravity) that causes the proppants to settle rapidly once suspended in a transporting fluid, e.g., fracture fluid or frac fluid. In particular, such proppants generally have a density above 1.60 g/cc. and often above 3.50 g/cc.
Specific gravity is defined as the ratio of the density of the material or substance whose specific gravity is being determined to the density of a reference material, usually water, reported at a reference temperature (usually under a condition where the density of water is 1 gm/cc). Specific gravity is a dimensionless quantity. The density of a material or substance is the ratio of the mass of the material to the volume that the mass of material occupies (mass/volume) and is often reported in grams/cubic centimeters (gm/cc or g/cc) or grams/milliliter (gm/ml or g/ml).
When the proppant settles too rapidly from the frac fluid, the settlement interferes with positioning of the proppant throughout the fractured formation. To counteract this result, the fracturing fluid is often thickened to increase its viscosity and thus slow the rate of a specific proppant's settlement. One issue with using a higher viscosity fracturing fluid, however, is the increased amount of energy required to pump the fluid into the subterranean formations when the fractures are created and when proppant is delivered throughout the formation. In other words, more energy is required to pump thicker fluids.
Another method of reducing the rate of proppant settlement is to use proppants having a lower specific gravity (i.e., a higher buoyancy) such as hollow glass balls, walnut hulls and sealed porous ceramics. These types of proppants exhibit a lower apparent specific gravity. The apparent specific gravity is the measurement of the specific gravity of a porous solid or substance when the volume used in the density calculation is considered to include the porosity, i.e., the porous permeable interior, of the porous solid or substance. Thus, in the case of porous materials the apparent density is less than the intrinsic density of just the solid mass of material.
Since these proppants appear or perform as if they are less dense than silica sand or ceramic proppants) they tend to settle more slowly in a fluid. However, these types of proppants are generally less crush resistant and realistically are only satisfactorily used in shallower wells at 3,000 to 4,000 psi closure pressures. Many formations may experience closure stresses of 6,000 to 10,000 psi and higher.
In view of the foregoing, interest in developing new solutions to proppant design and transport in well recovery operations remains strong. In particular, particulate compositions (proppants) that have slower settlement times yet are able to function at higher closure stresses continue to be in demand.